Sulfurized partial esters of polycarboxylic aromatic acids as lubricating oil additives



United States Patent SUIFURIZED PARTIAL FSTERS 0F POLYCAR- BOXYLIC AROMATIC ACIDS AS LUBRICATING OIL ADDITIVES Stephen 3. Metro, Scotch Plains, and Adolph J. Garbus, pnden, NJ., assignors to Esso Research and Engineerrng Company, a corporation of Delaware No Drawing. FilediNov. 19, 1958, Ser. No. 774,828

Claims. (Cl. 260-125) This invention relates to sulfurized partial esters of aromatic polycarboxylic acids and their anhydrides and to lubricating oil compositions containing said esters. Particularly, it relates to sulfurized partial esters prepared from mono or dihydric alcohols and aromatic acids having 3 to 4 carboxylic groups per molecule, which esters are useful as synthetic and mineral lubricating oil additives.

The use of various aliphatic diesters and complex synthetic esters as lubricating oils is well known to the art and has been described in numerous patents, e.g., US. 25

2,723,286; 2,743,234; and 2,575,196. The use of fully esterified triand tetra carboxylic acids as synthetic ester lubricating oils and the use of partial esters of triand tetra carboxylic acids as lubricating oil additives are disclosed in US. application Serial No, 706,593, filed January 2, 1958. This application discloses that the fully esterified derivatives of aromatic triand tetra carboxylic acids give synthetic lubricating oils which, in addition to having good viscosity properties at both high and low temperatures, also give lubricants which have extremely high load-carrying ability and which inhibit corrosion of lead bearings. The application further discloses that the partial esters of aromatic triand tetra carboxylic acids have pour points too high to be used as synthetic lubricants per se, but are valuable as load carrying and lead-corrosion-inhibiting additives in either synthetic or mineral lubricating oils.

It has now been found, and this finding forms the basis of the present invention, that if the partial esters of aromatic triand tetra carboxylic acids or their anhydrides are sulfurized and then added to lubricating oils, a surprising and unexpected increase in the load carrying ability of the oil will be obtained with no 4 reduction in the lead-corrosion-inhibiting properties of the oil-additive combination. Other advantages which have been found to be inherent in the case of the sulfurized partial ester additive of this invention are its increased solubility in ester-type lubricants and its high and low temperature stability.

The sulfurized partial esters of this invention include the sulfurized mono-, di-, and triesters of aromatic tetracarboxylic acids and the sulfurized monoand diesters of aromatic tricarboxylic acids and their anhydrides, which contain in the range of 0.5 to 6.4 wt. percent of sulfur. The sulfurized partial esters can contain in the range of .001 to 0.4, e.g., .001 to 0.12 wt. percent of mercaptan sulfur.

The partial esters of aromatic triand tetracarboxylic 2,985,645 Patented May 23, 1 961 acids which are useful in forming the sulfurized partial esters of this invention have the general formula: (H000),,,Ar(OO0R) wherein Ar represents a monocy'clic aromatic hydrocarbon ring having 6 carbon atoms, R represents an alkyl group having 3 to 20 carbon atoms, e.g. 5 to 13 carbon atoms, 11 is a number of from 1 to 3 and m is a number of from 1 to 3 such that m+n=3 if the acid is tribasic and m+n=4 if the acid is tetrabasic.

The partial esters of aromatic anhydrides are also useful in forming the sulfurized partial esters of this invention. Particularly these are compounds having the formula:

0 l5 Utmost o coon o wherein R represents an alkyl group having 3 to 20 and preferably 5 to 13 carbon atoms.

The aromatic polycarboxylic acids useful in preparing the above partial esters include the following:

C0 0]! COOH C 00H COOK (hernlmellitlc) (monoph C 00H H0 0 0 COOH COOK o H000 COOH COOK (pyromellitic) (prehnitic) H0 00 000K 000K where they exist, the corresponding partial or full acid anhydrides may also be used, e.g. pyromellitic acid anhydride which has the structure:

i i o/ \o Partial esters may also be prepared from the corresponding eyclohexane polycarboxylic acids which are formed when the above aromatic acids are catalytically' hydrogenated to saturate the ring. The sulfurized partial esters of this invention, however, are preferably prepared from pyromellitic acid or its anhydride since it is readily available in commercial quantities and undergoes esterification very readily.

The alcohols useful in forming the above partial esters are those alcohols having about 3 to 20, e.g 5 to carbon atoms per molecule. These alcohols may be either straight chain or branched chain alcohols. Primary alcohols are particularly preferred. Secondary and tertiary alcohols, while operable, are less preferred for the purposes of this invention, since partial esters prepared from such alcohols generally have poorer thermal stability than the corresponding esters prepared from primary alcohols. Among the straight chain primary alcohols operable in preparing the additives of this invention may be mentioned n-decyl alcohol, n-octyl alcohol, n-heptyl alcohol, n-hexyl alcohol, n-amyl alcohol, n-butyl alcohol, and Fischer-Tropsch synthesis alcohols. Operable branched chain primary alcohols include Z-ethylhexyl alcohol, 2- ethyl-butyl alcohol, and the 0x0 alcohols. These 0x0 alcohols are well known in the art. They are prepared in a two stage reaction. The first stage involves reacting olefins, such as polymers and copolymers of C and C monoolefins, with carbon monoxide and hydrogen at temperatures of about 300 F. to 400 F. and pressures of about 30 to 400 atmospheres, in the presence of a suitable catalyst, ordinarily a heavy metal carbonyl, such as cobalt carbonyl, to form a mixture of aldehydes having one carbon atom more than the olefin. In the second stage, the aldehyde mixture is hydrogenated, to form an isomeric mixture of highly branched chain primary alcohols which is recovered by distillation, The process is well known and has been described in various U. S. patents, e.g US. 2,327,066 and US. 2,593,428.

The aromatic acid partial esters can be prepared by two dilferent esterification techniques, depending upon the reactants employed. Preferably, the aromatic partial esters are prepared by reacting one or two moles of alcohol with 1 mole of an aromatic dianhydride. This reaction proceeds without catalyst and no water is evolved; the alcohol merely opens up the 2 anhydride rings. If a third mole of alcohol is reacted, then one mole of water is evolved. Whenever water is formed, water-entraining agents are normally used and the product must be heated until the water and water entraining agent are removed overhead. Where one or two .moles of alcohol are reacted with a dianhydride no water is formed, no solevnts are required and the reaction may be carried out by simply heating the reactants in an open vessel to temperatures in the range of 300 to 380 F. The reaction proceeds very rapidly after 180 F. has been reached. The reaction product is filtered while hot to give a clear, viscous fluid which has a waxy appearance upon cooling. The reaction between one mole of an aromatic dianhydride and two moles of alcohol is illustrated by the following equation:

H.000 GOOH acids with alcohol, more conventional esterification techniques are employed. This esterification is carried out by reacting one to three molar proportions of an alcohol with one molar proportion of the tetra carboxylic substituted aromatic acid, and one to two molar proportions of alcohol with one molar proportion of the tri-carboxylic substituted aromatic acid. Generally, a water-entraining agent, e.g., heptane, toluene, etc., is used and the reaction is carried on until the calculated amount of water is removed overhead. Esterification catalysts may be used, e.g., sodium bisulfate, sulfuric acid, toluene sulfonic acid, sodium methylate, calcium oxide, etc., although the reaction may be carried out without a catalyst. In any case after the desired amount of water is removed, the remaining reaction product will be filtered and washed if the catalyst was used, and distilled under vacuum in order to remove the entraining agent and any unreacted alcohol. Frequently, it is desirable to carry out this esterification reaction without a catalyst. This eliminates the necessity for filtering and washing to remove the catalyst which otherwise might tend to make the product corrosive.

The aromatic partial esters prepared by either by the above techniques are sulfurized by heating the partial ester and about 0.5 to 40.0 wt. percent of sulfur (based on the weight of ester employed) with stirring to temperatures of about 375 to 430 F. and preferably 390 to 420 F. for a period of time in the range of 15 to 75 minutes and preferably 20 to 60 minutes. At 300 F. the sulfur begins to dissolve and at 400 F. all the sulfur is either in solution or reacted. The sulfurized aromatic partial esters are then filtered while hot.

The finished lubricating oil compositions of this invention will comprise a major proportion of a lubricating oil and a minor amount, e.g., in the range of 0.01 to 5.0 wt. percent of the sulfurized partial ester additive of this invention Preferably, 0.1 to 1.0 wt. percent of the additive is used.

The lubricating oil used with the sulfurized partial ester additives of this invention may be a mineral lubricating oil, a synthetic lubricating oil, or any mixtures thereof, having a viscosity at 100 F, in the range of 10 to 50 cs. and a V.I. of about to 160. Particularly preferred synthetic oils for blending with the aromatic acid ester materials are the saturated aliphatic diesters represented by the formula:

wherein R is a straight or branched chain hydrocarbon radical of a C, to C alkandioic acid, while R' represents an alkylradical of a C to C branched or straight chain alkanol, and the total number of carbon atoms in the molecule is in the range of 20 to 36 Specific examples of such diesters include di-(2-ethylhexyl) sebacate, di-(C 0x0) adipate, etc. Other synthetic oils which may be used will include esters of monobasic acids (e.g. C Oxo alcohol ester of 0 0x0 acid), esters of glycols (e.g. C Oxo acid diester of tetraethylene glycol), complex esters, esters of phosphoric acid, halocarbon oils, sulfite esters, silicone oils, carbonates, formals, polyglycol-type synthetic oils etc. Various other additives may also be added to the lubricating compositions of the invention in amounts of about 0.1 to 10.0 weight percent, based on the total weight of the composition. For example, detergents such as calcium petroleum sulfonate; oxidation inhibitors such as phenyl-alpha-naphthylamine or phenothiazine; corrosion inhibitors such as sorbitan monooleate; pour point depressants; dyes; grease thickeners; other load-carrying agents and the like may be added.

The invention will be further understood by the following examples: EXAMPLE I 6 a be tested in 100 cc. of the sample maintained at 347' F. for 72 hours while bubbling 0.5 liter per hour of air through the sample. The metal strip is then reweighed to determine the weight change as mg./cm.'

The load-carrying test consisted of conducting a Ryder gear test in accordance with MlL-b-7808C specification procedure.

The lead corrosiveness and load-carrying ability of a non-sulfurized di-(C Oxo) ester of pyromellitic acid were also tested in accordance with the above procedure.

The compositions tested and the results obtained are summarized in the following table:

Table I Pb Corr. Wt. Test-Oxidation-Oorr. at Wt. Per- Loss in 347 F. (mgJcmJ) Ryder Gear cent Ad- (mg/in!) Test (Load Additive ditive in in Lbs.)

Base 011 After After Cu Mg Fe Al Ag 9Hrs. tHrs.

None None 3- 8 15. 2 +0. 0 0. 01 0. 01 0. 0 0. 0i 2, Bullurized PMDA Hall Eaten-.. 0. 50 0. 0 0. 0 +0. 31 -0. 03 0. 04 -0. 02 0. 08 3, 060/2, 920 Non-Snlturized PMDA Halt Eaten. 0. 0.0 0. 41 Do 0.60 2,345 Do 1. 0 0. 0 0. 0 2, 390 MIL-b78080 Specifications for 0:. Corr. at

347 F. (mg/cm!) 5:0.4 =t=0. 2 5:0.2 =t=0.2 $0.2

colored liquid reaction product is formed. The reaction is illustrated by the following equation:

The above diester was then sulfurized by mixing the diester with 0.5 gram (1.0 wt. percent based on the weight of the diester) of purified flowers of sulfur. The sulfurization was carried out by heating the diester and sulfur in an open flask for minutes at a temperature of 400 F. The sulfurized product contained 0.8 wt. percent sul- HO 0 C C OOH ROOC COOR fur.

(B) 0.5 wt. percent of the above sulfurized partial ester was then blended with 99.5 wt. percent of a base oil which consisted of a 50:50 (by volume) mixture of di-(C, Oxo) adipate and di-(C Oxo) adipate and 0.5 wt. percent of phenothiazine as an oxidation inhibitor. The C, 0x0 groups are derived from C, Oxo alcohols prepared by subjecting C (butylene-propylene) olefins to the 0x0 process. The C 0x0 groups are derived from C Oxo alcohols prepared by subjecting C, (tripropylene) olefins to the 0x0 process.

Since synthetic oils are frequently used to lubricate lead-containing bearings, the base oil and the above blend were tested for lead corrosiveness, for corrosiveness to other metals, and for load-carrying ability.

The lead corrosion test was carried out by rapidly rotating a bimetallic strip consisting of a lead strip and a copper strip bound together, in an oil sample maintained at 325 F. While air is bubbled through the sample. The weight loss of the lead strip is then determined, and reported in terms of mg. wt. loss per square inch of lead surface. Lubricants possessing a very low lead corrosiveness are desired for aircraft engine lubrication.

The test for corrosion to other metals was carried out in accordance with MIL-L-7808C specification procedure, i.e., by immersing weighed strips of the metal to furized partial esters of this invention meet the specifications of MIL-L 7808C as regards the corrosiveness of the oxidized oil (heated to 347 F.) to other metals.

EXAMPLE 11 Additional data on the di-(C, Oxo) sulfun'zed partial esters (sulfurized PMDA half esters) of Example I (A) were obtained by forming the ester and sulfurizing it with'diifcrent amounts of sulfur according to the method of Example I (A). The composition of the sulfurized partial esters and the reaction conditions for the sulfurization are given in Table II. These sulfurized partial esters were added in concentrations of 0.5 wt. percent to the base oil of Example I (B) and the resultant lubrieating oil blends were tested for lead corrosiveness and corrosiveness to other metals according to the test procedures of Example I. The results are given in Table II.

The sulfurized partial esters of this example were also blended in concentrations of'0.25 wt. percent and 0.75 wt. percent, respectively, with the base oil of Example I (B) and tested for load-carrying ability. The test used was the SAE-SOD load test, which measures the load-carrying ability of a lubricating oil by use of a standard SA'Ev lubricants tester, having a gear ratio of 3.4 to 1 in place of the conventional gear ratio of 14.6 to 1. The testwas carried'out by running the machine for two minutes imder a 50-p0und load and then manually increasing the load 50 pounds every 10 seconds aesaese 7 until scufling occurred. The results are also given in Table II.

Table II Sull'urized PMDA Halt Ester Composition of Sulfurized PMDA Additives 1-4 Halt Ester Additives Gm. ot PMDA Halt Ester 34. 8 34. 3 34. 8 34. 8 Gm. of Sulfur 3. 2 6. 4 9. 6 12. 8 Wt. Percent Sulfur Adde 9. 2 18. 4 27. 6 36. 8 Reaction ime, 34 45 30 25 Temperature, F 400 410 410 400 Percent Mercaptan Sulfur" N ll Nil Nil Nil Percent Su ur 3. 90 4. 27 4. 46 6. 11 Base Oil of Example I(B)+0.5 wt.

Percent of Additives Hi:

Oxidation-Corr. Test at 347 F.

(mg./cm)- 0 0 0 0 0 0 After 12 his 0 0 Base Oil 1 of Example I(B)+Additives 1-4:

BAESOD Load Test (in Lbs.)-

Wt. Additive:

1 Base 011 without additives carried 450 lbs. in the GAE-SOD load.

EXAMPLE III A second di-(C Oxo) ester of pyromellitic acid was prepared as follows: Approximately one mole of the di- (C 0x0) ester of pyromellitic acid was placed in a flask equipped for removal of entrained water overhead. ml. of heptane were added to act as a water entrainer and the mixture was slowly heated to a temperature of about 518 F. The temperature was maintained at about 518 F. for more than 2 hours and the total reaction time was about 6 to 7 hours. One mole of water was formed and the water and heptane were separated overhead from the diester product. The reaction may be illustrated by the following formula:

This ester was then sulfurized by adding 0.5 gram of sulfur to 50 grams of the diester in an open beaker and heating for minutes at a temperature of 400 F. The sulfurized product contained 0.04 wt. percent mercaptan sulfur and 0.92 percent of combined sulfur. This sulfurized partial ester was then blended with the base oil of Example I in concentrations of 0.25 wt. percent and 0.75 wt. percent, respectively, and tested for load-carrying ability by means of the SAE-SOD load test of Example 11. Th base oil containing 0.25 wt. percent of the sulfurized partial ester carried 550 pounds. The base oil containing 0.75 wt. percent of the partial ester carried 500 pounds.

O HO-ii RO-C 8 EXAMPLE IV A. A quarteror monoester is prepared by reacting 1 molar amount of C Oxo alcohol (prepared from tetrapropylene by the 0x0 process) with 1 molar amount of pyromellitic dianhydride. This ester is prepared according to the procedure of Example I.

The monoester is then sulfurized by treating wt. percent of the ester with 4 wt. percent of sulfur (based on the weight of the monoester) at 400 F. for half an hour.

B. As an example of the use of the sulfurized partial esters of this invention in mineral oil, 0.25 wt. percent of the above monoester is added to 99.75 wt. percent of a paraflinic mineral lubricating oil having a viscosity of 10 cs. at 100 F. and a viscosity index of 106.

EXAMPLE V A. A triester is prepared by reacting 3 molar proportions of an amyl alcohol with 1 molar proportion of prehnitic acid according to the procedure of Example III, (i.e., heptane is added as a water eutrainer).

'Ihe triester is then sulfurized by treating 100 wt. percent of the ester with 1.0 wt. percent of sulfur at 400 F. for half an hour.

B. As an example of the use of the sulfurized partial esters of this invention in different synthetic oils, 0.75 wt. percent of the above sulfurized triester is added to 99.25 wt. percent of a synthetic diester base lubricating composition. This diester base lubricant consists of 949+ parts by weight of di-(C 0x0) adipate; 3 parts by weight of a complex ester prepared by simultaneously reacting 1 mole of polyethylene glycol of 200 mol. wt., 2 moles of C 0x0 alcohol and 2 moles of adipic acid; 1 part by weight of tricresyl phosphate; 1 part by weight of phenothiazine and .003 part by weight of an antifoamant.

What is claimed is:

1. As a lubricating oil additive a compound selected from the group consisting of the sulfurized partial alkyl estersof monocyclic aromatic hydrocarbon triand tetracarboxylic acids and their anhydrides containing in the range of 0.5 to 6.4 weight percent of sulfur, and having about 3 to 20 carbon atoms per alkyl group.

2. The lubricating oil additive according to claim 1, where said sulfurized partial alkyl esters are prepared by reacting a compound selected from the group consisting of the partial alkyl esters of monocyclic aromatic hydrocarbon triand tetracarboxylic acids and their anhydrides with about 0.5 to 40.0 weight percent of sulfur, based on the weight of said partial ester, at temperatures in the range of 375 to 430 F. for a period in the range of 20 to 60 minutes.

3. The lubricating oil additive according to claim 2, wherein said partial alkyl esters have the general formula:

(HOOC) Ar(COOR) where Ar represents a monocyclic aromatic hydrocarbon ring having 6 carbon atoms, R represents an alkyl group having 5 to 13 carbon atoms, n is a number of 1 to 3 and m is a number of 1 to 3 such that m+n=3 when the acid is tribasic and m+n=4 when the acid is tetrabasic.

4. As a lubricating oil additive a sulfurized partial alkyl ester prepared by reacting a compound of the general formula:

COOK

wherein R is an alkyl group having from to '13 carbon atoms, with from about 0.5 to 40 weight percent of sulfur, based on the weight of said compound, at temperatures in the range of from 375 to 430 F. for a period of from 20 to 60 minutes, said sulfurized partial ester containing in the range of 0.5 to 6.4 weight percent sulfur.

5. As a lubricating oil additive a sulfurized partial alkyl ester prepared by reacting, at temperatures in the range of 300 to 380 F., .one molar proportion of pyromellitic dianhydride with from 1 to 2 molar proportions of an aliphatic alcohol having from 3 to 20 carbon atoms per molecule and then reacting the resulting partial alkyl ester with from about 0.5 to 40 weight percent of sulfur, based on the weight of said partial ester, at tem- V 10 peratures in the range of from 375' to 430 F. for a period of from to minutes, said sulfurized partial ester containing in the'range of 0.5 to 6.4 weight percent sulfur.

References Cited in the file of this patent UNITED STATES PATENTS 

1. AS A LUBRICATING OIL ADDITIVE A COMPOUND SELECTED FROM THE GROUP CONSISTING OF THE SULFURIZED PARTIAL ALKYL ESTERS OF MONOCYCLIC AROMATIC HYDROCARBON TRI- AND TETRACARBOXYLIC ACIDS AND THEIR ANHYDRIDES CONTAINING IN THE RANGE OF 0.5 TO 6.4 WEIGHT PERCENT SULFUR, AND HAVING ABOUT 3 TO 20 CARBON ATOMS PER ALKYL GROUP. 